Photoelectric current multiplier using molecular crystal and production method therefor

ABSTRACT

A NTCDA single crystal is used as a photoelectric current multiplier layer, and Au thin films are formed as electrodes on the opposite surfaces of the multiplier layer by a vapor deposition method to form a sandwich type cell. When a voltage is applied to the NTCDA single crystal by the electrodes from a dc power source and a monochromatic light is applied, a multiplied photoelectric current flows between the electrodes. A rise of this element at light-on is considerably faster than when a vapor-deposited layer is used as a photoelectric current multiplier layer to permit a faster response.

TECHNICAL FIELD

The present invention relates to an organic photo-electronics apparatus,and specifically, to a photoelectric current multiplier including aphotoelectric current multiplier device which utilizes a photoelectriccurrent multiplication phenomenon at the organic/metallic interfacecaused by a photoconductive organic semiconductor, and a light-to-lighttransducer further equipped with an organic electroluminescent (organicEL) layer for obtaining light-to-light transduced light, and aproduction method thereof.

The photoelectric current multiplier is applicable to a photo sensor andthe like.

DESCRIPTION OF THE RELATED ART

Conventionally, a photoelectric current multiplier utilizing such amultiplication effect of photoelectric current at the organic/metallicintei-face had a sandwich-type cell structure in which a vapor-depositedthin film formed of an organic semiconductor is sandwiched between twometallic electrodes. As a specific example, there are known thoseadopting a vapor-deposited thin film formed of metal-substitutedphthalocyanine. (Me-PTC) and of a perylene pigment as an organicsemiconductor (See for example, M. Hiramoto, T. Imahigashi, and M.Yokoyama, Applied Physics Letters.64, 187 (1994)), those adopting avapor-deposited thin film of naphthalene tetracarboxylic anhydride(NTCDA) which is a naphthalene derivative (See for example, T. Katsume,M. Hiramoto, and M. Yokoyama, Applied Physics Letters.69, 3722 (1996))and the like.

The aforementioned conventional photoelectric current multiplier devicebased on the vapor-deposited thin film has a multiplication factor(ratio of number of electrons by the multiplied photoelectric currentflowing in the device, relative to the number of incident photons)reaching 10⁵-fold, and hence provides light detectability which iscomparable to that of a photoelectron multiplier currently used forlight detection. Therefore, the aforementioned conventionalphotoelectric current multiplier has sufficient potentiality as a lightsensing device.

This multiplication phenomenon occurs by tunneling injection ofelectrons from the metal of electrode, induced by accumulation of lightgeneration holes to the organic/metallic interface, as shown in theenergy state chart of FIG. 1. In FIG. 1, the vertical axis representselectron energy, the open circle denotes a hole, and the solid circledenotes an electron. In this case, NTCDA is used as the organicsemiconductor and gold (Au) is used as the electrodes adjoining to theorganic semiconductor.

It is already known that the organic/metallic interface trap whichaccumulates the holes to cause multiplication is a dead end structure(structural trap) resulting from nonuniform adherence due to incompletejoining between the organic thin film (in this case NTCDA) and themetal, as shown in the model of FIG. 2.

However, in the conventional photoelectric current multiplier whichadopts a vapor-deposited thin film using an organic pigment (forexample, Me-PTC which is a perylene pigment and NTCDA which is anaphthalene derivative), a time in the order of several tens of secondsis required for responding to the starting of light application(light-on) and ending of light application (light-off) of the multipliedphotoelectric current, leading to the drawback that the light responseof multiplied photoelectric current is very slow. This drawback hashindered application of the photoelectric current multiplier as a photosensor.

It is an object of the present invention to increase the response speedof the photoelectric current multiplier.

DISCLOSURE OF THE INVENTION

For establishing an alternative to the conventional photoelectriccurrent multiplier which adopts a vapor-deposited thin film of anorganic pigment, the inventors of the present application attempted touse a single crystal of an organic pigment in place of thevapor-deposited thin film of an organic pigment. Precedents of aphotoelectric current multiplier using a single crystal of organicpigment had not been found.

The photoelectric current multiplier of the present invention compriseselectrodes for applying a voltage to a photoelectric current multiplierlayer containing a photoconductive organic semiconductor, thephotoconductive organic semiconductor being formed of a single crystalof an organic pigment, wherein a light irradiation-induced current canbe obtained with a quantum yield multiplied by 1-fold or more by lightirradiation in the condition that a voltage is applied by theelectrodes, thereby realizing faster response than a vapor-depositedfilm formed of the same material.

In the conventional photoelectric current multiplier using avapor-deposited thin film of an organic pigment, the photoelectriccurrent multiplication phenomenon is known to depend largely on thecondition of the organic semiconductor at the interface with the metal.For this reason, it has been impossible to predict whether photoelectriccurrent multiplication phenomenon that was observed in a vapor-depositedthin film also occurs in a single crystal.

With regard to the multiplication phenomenon in an organic singlecrystal, observation has been reported once (H. Kallman and M. Pope,Proceeding of symposium on electrical conductivity in organic solids,pp.21-25, Duke University, Durham, N.C., Apr. 20-22, 1960). However,this report described no more than a single crystal of organic materialbeing used as the material, and did not proved the mechanism ofmultiplication. Therefore, this report gives no suggestion about thekinds of materials that show photoelectric current multiplication amongan infinite number of organic materials. This can be acknowledged fromthe fact that no reports have been issued about photoelectric currentmultiplication using a single crystal of organic material in the 40years since this report was published.

The significant feature of the present invention is: finding that theresponse becomes faster than that of the photoelectric currentmultiplier using a vapor-deposited film of an organic pigment, as wellas confirming that photoelectric current multiplication phenomenonoccurs by using a single crystal of an organic pigment as aphotoconductive organic semiconductor layer.

In the case where a single crystal organic pigment is used in thephotoconductive organic semiconductor layer, the degree of transfer ofcarriers becomes much larger than that of a vapor-deposited film due tothe fact that the single crystal does not have grain boundaries, andhence it is conceivable that the response is faster.

There are also reports of a light-to-light transducer in which anorganic electroluminescent (organic EL) layer is laminated andintegrated to a photoelectric current multiplier layer formed by aphotoconductive organic semiconductor, for converting wavelength oflight and multiplying the light (See T. Katsume, M. Hiramoto, and M.Yokoyama, Appl. Phys. Lett., 64, 2546 (1994), and M. Hiramoto, T.Katsume, and M. Yokoyama, Opt. Rev., 1, 82 (1994)). The photoelectriccurrent multiplier layer of these reports is a vapor-deposited thin filmof a photoconductive organic semiconductor. However, when thephotoelectric current multiplier layer is replaced with a single crystalof an organic pigment in accordance with the present invention, alight-to-light transducer can be constructed in the similar manner.

In other words, the fast responsive photoelectric current multiplier ofthe present invention also includes a light-to-light transducer whichobtains as an output light-to-light transduced light from the organicelectroluminescent layer by converting the light irradiation-inducedcurrent into light rather than obtaining light irradiation-inducedcurrent, since the organic electroluminescent layer is laminated andintegrated to the photoelectric current multiplier layer and thephotoelectric current multiplier layer is irradiated with light.

It is anticipated that multiplication characteristics such asmultiplication factor and response speed will be desirably controlled ifit is possible to flexibly control the ultrafine structure of theorganic thin film. In the past, however, it was impossible to observethe structure of the organic vapor-deposited thin film, which is anassembly of fine crystals, in molecular level. For this reason, therelationship between the ultrafine structure of organic side and thestructural trap was unclear, so that it was impossible to undertake thecontrol of multiplication characteristics by controlling the ultrafinestructure at the organic/metallic interface.

The inventors of the present invention succeeded in controlling themultiplication characteristics by way of controlling the structure ofsingle crystal surface in molecular level by using a single crystal asthe photoconductive organic semiconductor.

The photoelectric current multiplier device using the vapor-depositedthin film as described above is incapable of controlling the structureof the organic thin film in molecular level. However, by using anorganic semiconductor single crystal, it becomes possible to control themultiplication characteristics by way of controlling the structure ofcrystal surface in molecular level.

Another aspect of the photoelectric current multiplier of the presentinvention is a photoelectric current multiplier wherein multiplicationcharacteristics are controlled by way of controlling the structure of anorganic/metallic interface created by using a single crystal of anorganic semiconductor in molecular level.

According to a production method of the present invention, in producinga photoelectric current multiplier in which a light irradiation-inducedcurrent is obtained with a quantum yield multiplied by irradiating aphotoelectric current multiplier layer with light in the condition thata voltage is applied to the photoelectric current multiplier layercontaining a photoconductive organic semiconductor, a single crystal isadopted as the photoconductive organic semiconductor, and photoelectriccurrent multiplication characteristics are controlled by controllingmolecular step structure of the single crystal at an interface betweenthe single crystal and the metal of electrode.

In the photoelectric current multiplier of the present invention, sincethe organic semiconductor layer of the photoelectric current multiplierlayer is a solid single crystal, it is possible to form an electrodethin film on the organic semiconductor layer by, for example, a vapordeposition technique. As a result, in contrast to the case where anorganic semiconductor vapor-deposited film is used as the photoelectriccurrent multiplier layer, a substrate for supporting electrode layersand photoelectric current multiplier layer is no longer needed.Furthermore, there is an advantage that a variety of electrode materialscan be selected at will regardless of the compatibility between anelectrode material on a substrate and an organic semiconductor thin filmto be deposited thereon, which is problematic in the case ofvapor-deposited film.

Furthermore, since there is no need to provide a supporting substrate incontrast to the vapor-deposited film, produce of a perfectly symmetricaldevice is enabled.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an energy structure view at an NTCDA/Au interface at the timeof multiplication;

FIG. 2 is an imaginary view of a structural trap at the organic/metallicinterface;

FIG. 3 describes chemical formulas showing several examples ofphotoconductive organic semiconductors used in the present invention;

FIG. 4 describes chemical formulas exemplifying a compound used as anorganic electroluminescent layer and a compound used as a carriertransfer layer when the present invention is applied to a light-to-lighttransducer;

FIGS. 5A and 5B are schematic section views each showing a photoelectriccurrent multiplier of an embodiment of the present invention;

FIG. 6 is a schematic section view showing an apparatus formanufacturing an organic semiconductor single crystal in one embodiment;

FIGS. 7A and 7B show two kinds of NTCDA single crystals manufactured inthe same embodiment; FIG. 7A shows an NTCDA single crystal manufacturedunder reduced pressure condition, and FIG. 7B shows an NTCDA singlecrystal manufactured under atmospheric pressure condition;

FIG. 8 shows the data of dependence on applied voltage of multiplicationfactor measured with regard to photoelectric current multipliersmanufactured by using two kinds of NTCDA single crystals (a) and (b)produced in the same embodiment;

FIG. 9 are views showing wavelength-dependence of multiplication factorand absorbance spectrum of an NTCDA thin film in the same embodiment;

FIG. 10 is a view showing an AFM image (200 nm×200 nm) of a goldelectrode which is vapor-deposited in a thickness of 20 nm on thesurface of NTCDA single crystal;

FIG. 11A and FIG. 11B are views showing AFM images of the two kinds ofNTCDA single crystals manufactured in the same embodiment. FIG. 11Ashows an NTCDA single crystal manufactured under reduced pressurecondition, and FIG. 11B shows an NTCDA single crystal manufactured underatmospheric pressure, and in both cases, the upper image shows thesurface condition of an area of about 1 μm×1 μm observed as a planeimage, and the lower image shows the bumpy condition of the surfacewhile part of the upper image is cut in the transverse direction;

FIG. 12 is a view showing a molecular dead-end road model in the sameembodiment;

FIG. 13 is a graph showing dependence on applied voltage ofmultiplication factor exhibited by the crystalline Me-PTCvapor-deposition thin film and an amorphous PhEt-PTC vapor-depositionthin film;

FIG. 14 has been canceled.

FIGS. 15A and 15B are waveform charts showing a comparison of responsecharacteristics in the same field intensity in embodiment using NTCDAsingle crystal (FIG. 15A) and in comparative embodiment using NTCDAvapor-deposited film (FIG. 15B);

FIG. 16 is a view showing dependence on applied voltage ofmultiplication factor exhibited by the photoelectric current multipliercreated using an NTCDA single crystal manufacture in other embodiment,wherein (c) uses a sample in which NTCDA is vapor-deposited on thesurface of NTCDA single crystal substrate by the same embodiment, and(b) uses an NTCDA single crystal manufactured under atmospheric pressurecondition;

FIG. 17 is a view showing an AFM image of the surface of the samplewherein NTCDA is vapor-deposited on the NTCDA single crystal substratemanufactured under atmospheric pressure condition; and

FIG. 18A is a view showing an AFM image of 200 nm×200 nm on the surfaceof an NTCDA single crystal; FIG. 18B is an enlarged view in which thekinking part of FIG. 18A is marked, and FIG. 18C is a view schematicallyshowing the size of an NTCDA molecule.

BEST MODE FOR CARRYING OUT THE INVENTION

As a single crystal that can be used for an organic semiconductor layerof a photoelectric current multiplier layer, single crystals of organicsemiconductors as mentioned below which exert photoelectric currentmultiplying function by a vapor-deposited film can be exemplified. Someof such single crystals are exemplified in FIG. 3. That is,phthalocyanine-based pigments [phtalocyanine and its derivatives (MPchaving various kinds of metals in the center; H₂Pc not having metals inthe center, those having various kinds of substituents)],quinacridone-based pigments (quinacridone (DQ) and its derivatives),perylene-based pigments [perylene and its derivatives (various kinds ofderivatives having different substituents on the nitrogen atom areknown, for example, t-BuPh-PTC, PhEt-PTC and the like, and Im-PTC havinghigh photoelectric transducing ability)], naphthalene derivatives(wherein the perylene backbone of a perylene pigment is substituted bynaphthalene, for example NTCDA) and the like. However, the singlecrystals of photoconductive organic semiconductor that can be utilizedin the present invention are not limited to the above, and singlecrystals such as pentacene and its derivatives, porphyrin and itsderivatives, merocyanine and its derivatives, C60 (fullerene) and thelike may be used.

In constructing a light-to-light transducer, as the organicelectroluminescent layer, vapor-deposited films of aluminum-quinolinolcomplex (Alq3) (denoted by the symbol “C20” in FIG. 4), 3, 4, 9,10-perylene tetracarboxylic 3,4:9,10-bis(phenylethylimide) and the likecan be exemplified.

The film thickness of the organic electroluminescent layer isappropriately from 0.05 to 0.1 μm.

The light-to-light transducer may be provided with a carrier transferlayer (a hole transfer layer or an electron transfer layer) between theorganic electroluminescent layer and the electrode. Vapor-depositedfilms of triphenyl-diamine derivatives (TPD) such asN,N-diphenyl-N,N-bis(4-methylphenyl)-4,4-diamine (denoted by the symbol“C21” in FIG. 4), 3,5-dimethyl-3,5-ditertiary butyl-4,4-diphenoquinone,2-(4-biphenyl)-5-(4-tertiary butylphenyl)-1,3,4-oxadiazole,N,N,N,N-tetra-(m-toluyl)-m-phenylene diamine and the like can beexemplified as the carrier transfer layer.

The film thickness of the carrier transfer layer is appropriately from0.05 to 0.1 μm.

With regard to the electrodes, a vapor-deposited film or a sputteredfilm made of gold or other metal may be used besides an ITO (indium tinoxide) transparent electrode as the electrode film provided on the sidewhere transparency is required. The electrode films can be formed byvapor-deposition or sputtering technique on the photoelectric currentmultiplier layer or a laminate of the photoelectric current multiplierlayer and the organic electroluminescent layer.

FIG. 5A schematically shows the section of one embodiment of aphotoelectric current multiplier of the present invention. On both sidesof a photoconductive organic semiconductor single crystal 2 which is aphotoelectric current multiplier layer, electrodes 4, 6 are formed toconfigure a sandwich-type cell. A DC power source 8 applies a voltage tothe organic semiconductor single crystal 2 by means of the electrodes 4,6.

As one example, an NTCDA single crystal with a thickness of 167 μmmanufactured under reduced pressure condition as described later wasused as the organic semiconductor single crystal 2, and an Au thin filmhaving an thickness of 20 nm formed by vapor deposition was used as theelectrodes 4, 6. As the DC power source 8, a power source capable ofapplying voltages of up to 1,000V to the organic semiconductor singlecrystal 2 was arranged.

By irradiating the organic semiconductor single crystal 2 with light 18while a voltage from the power source 8 is applied on the organicsemiconductor single crystal 2 by the electrodes 4, 6, photoelectriccurrent multiplication occurs at the interface between the organicsemiconductor single crystal 2 and the electrode 4. Multiplicationfactor (photoelectric current quantum yield) was calculated by dividingthe number of carriers having flown as the photoelectric current by thenumber of photons absorbed by the organic semiconductor single crystal2.

When the present invention is applied to a light-to-light transducer, anorganic electroluminescent layer is laminated and integrated on theorganic semiconductor single crystal 2. One example of cell structure insuch a case is schematically shown in FIG. 5B. An organicelectroluminescent layer 10 is laminated and integrated on the organicsemiconductor single crystal 2. In this laminate, the electrode 4 isprovided on the organic semiconductor single crystal 2, and theelectrode 6 is provided on the organic electroluminescent layer 10 via ahole transfer layer 11. It is preferred to interpose the hole transferlayer 11 between the organic electroluminescent layer 10 and theelectrode 6 as illustrated. By irradiating the organic semiconductorsingle crystal 2 with the light 18 while applying a voltage from thepower source 8 to the laminate of the organic semiconductor singlecrystal 2 and the organic electroluminescent layer 10 via the holetransfer layer 11 by means of the electrodes 4, 6, light-to-lighttransduced light 19 having a wavelength which is different from that ofthe radiation light 18 from the organic electroluminescent layer 10 isgenerated.

In this light-to-light transducer, since the radiation light 18 isapplied to the organic semiconductor single crystal 2 via the electrode4, the electrode 4 should be permeable to the radiation light 18.Furthermore, since the light-to-light transduced light 19 is drawn outfrom the organic electroluminescent layer 10 via the hole transfer layer11 and the electrode 6, the hole transfer layer 11 and the electrode 6should be transparent to the light 19 thus generated.

The photoelectric current multipliers as described above can bemanufactured by laminating each layer on the organic semiconductorsingle crystal 2.

The organic semiconductor single crystal was manufactured by using acrystal growing apparatus as shown in FIG. 6. This crystal growingapparatus is configured by disposing a crystal growing tube 24 in areaction tube 22 of an electric furnace 20, and disposing a materialtube 26 on the upper side in the tube 24 (left side in the drawing). Inthe material tube 26 is exposed organic semiconductor powder 28 as amaterial. The reaction tube 22, crystal growing tube 24 and materialtube 26 are made of quartz glass.

It is so configured that carrier gas can flow from the upstream side ofthe reaction tube 20. As the carrier gas, any gases that are notreactive with the components of the organic semiconductor to be grown,as well as inert gases such as nitrogen, argon and helium can be used.

The electric furnace 20 disposed outside the reaction tube 20 consistsof three electric furnaces connected so as to be divided into threeareas 20 a to 20 c along the longitudinal direction of the reaction tube22. The area 20 a is an area where the material 28 is heated, the area20 b is a crystal growing area disposed downstream of the area 20 a, andthe area 20 c is an area disposed downstream of the area 20 b. Theelectric furnaces in the areas 20 a to 20 c are respectively providedwith thermocouples 30 a to 30 c, to allow independent temperaturecontrol for the respective areas 20 a to 20 c. The temperature of theelectric furnace 20 was set to have a temperature gradient decreasingtoward the downstream side (right side in the drawing) from the upstreamside (left side in the drawing).

As one example, an NTCDA was allowed to grow. In this case, thetemperature of the electric furnace 20 was set so that it is 330° C. atthe position of the thermocouple 30 a, 200° C. at the position of thethermocouple 30 b, and 100° C. at the position of the thermocouple 30 c.NTCDA powder of the material 28 was placed at the portion of 330° C. Thesingle crystal was precipitated in the crystal growing tube 24 havinglower temperature.

In growing of the NTCDA single crystal, the single crystal wasmanufactured in two different flow gas pressure conditions with regardto the carrier gas. First the single crystal was manufactured in reducedpressure condition (about 1 Torr) under nitrogen flow (also referred toas reduced pressure condition), and secondly the single crystal wasmanufactured in one atmospheric pressure under nitrogen flow (alsoreferred to as atmospheric pressure condition). The portion where theNTCDA single crystal precipitated had a temperature of about 240° C. inboth pressure conditions.

FIG. 7A shows a photograph image of the NTCDA single crystalmanufactured under reduced pressure condition. This single crystal has aplate shape having a thickness of typically about 200 μm.

FIG. 7B shows a photograph image of the NTCDA single crystalmanufactured under atmospheric pressure condition. This single crystalhas an elongated plate shape having a thickness of typically 30 to 160μm.

It was thus demonstrated that the crystal shape of single crystaldiffers depending on the pressure condition at the time ofmanufacturing.

A photoelectric current multiplier shown in FIG. 5A was configured byusing these two kinds of NTCDA single crystals as a photoconductiveorganic semiconductor crystal. The electrodes 4, 6 are Auvapor-deposited thin films having a thickness of 20 nm. The thickness ofthe NTCDA single crystal manufactured under reduced pressure conditionwas 167 μm.

FIG. 8 shows dependence of the multiplication factor on applied electricfield when each photoelectric current multiplier was irradiated from theside of the negative electrode 4 with a monochromatic light of 380 nm asthe radiation light 18. Since the thickness differs in crystal samples,a comparison was made in the same electric field intensity conditionwhile representing the horizontal axis by an electric field intensitywhich is obtained by dividing an applied voltage by a thickness ofcrystal. Multiplication factor (photoelectric current quantum yield) wascalculated by dividing the number of carriers having flown as thephotoelectric current by the number of photons absorbed by the NTCDAsingle crystal.

These results showed that photoelectric current multiplicationphenomenon occurs in the single crystalline NTCDA. However themultiplication factor perfectly differs depending on the manufacturingcondition of the crystal. That is, the single crystal manufactured underreduced pressure condition showed 200-fold multiplication factor at theelectric field intensity of 4×10⁴V/cm ((a) in FIG. 8), whereas thesingle crystal manufactured under atmospheric pressure condition showedas small as about 10-fold multiplication factor even at the electricfield intensity of 1×10⁵V/cm ((b) in FIG. 8).

Incidentally, the multiplication factor of the vapor-deposited film caneasily be increased to several ten-thousands fold. This is because thevapor-deposited film has a thickness of about 500 nm in contrast to thesingle crystal 2 of the present embodiment having a thickness of 167 μm,so that the electric field intensity exerted on the organicsemiconductor is significantly larger in the vapor-deposited film.

If a thinner single crystal is obtainable as a result of considerationof manufacturing condition of the single crystal, multiplication factorwhich is near to that of the vapor-deposited film will be obtained.

FIG. 9 shows wavelength-dependence of multiplication factor togetherwith absorbance of NTCDA. When light radiation was conducted from theside of negatively-biased electrode, multiplication factor of not lessthan 20-fold was obtained at the absorption peak of NTCDA.

To the contrary, when light radiation was conducted from the positiveside, almost no multiplication was observed. This result clearly showsthat multiplication is a phenomenon occurring at the interface betweenthe negative-side Au electrode and the crystal. This is substantiallyequal to the multiplication behavior of the NTCDA thin film manufacturedby vacuum vapor deposition, concluding that multiplication according tothe light-induced electron injection mechanism shown in FIG. 1 wasobserved.

As described above, since multiplication phenomenon also occurs in theNTCDA single crystal, a structural trap which causes multiplication mustbe present at the NTCDA crystal/Au interface.

Next, observation results of the ultrafine structure of the interfaceshall be described.

FIG. 10 shows an atomic force microscope (AFM) image of the Auvapor-deposited electrode vapor-deposited on the NTCDA single crystal.It was found that Au is an assembly of ultrafine particles of about 20nm in diameter. Since Au particles have a spherical shape, there is aspatial gap at the interface between the single crystal and Au.

FIGS. 11A and 11B show atomic force microscope (AFM) images of surfaceof the respective single crystals. FIG. 11A shows the NTCDA singlecrystal manufactured under reduced pressure condition, and FIG. 11Bshows the NTCDA single crystal manufactured under atmospheric pressurecondition. Upper image of each Figure shows the surface condition ofabout 1 μm×1 μm area observed in a plane image. Lower image of eachFigure shows a section view of part of the above image cut in thetransverse direction and shows the bumpy condition of the surface. FIGS.11A and 11B are given in different units on the horizontal axis as FIG.11A illustrates a range of a little over 100 nm and FIG. 11B illustratesa range of a little under 200 nm.

As shown in FIG. 11A, many step structures were observed on the surfaceof the single crystal manufactured under reduced pressure condition, andthe step difference of each step was about 0.5 nm which is a width ofthe NTCDA molecule. This step structure is called a molecular stepstructure. To the contrary, as shown in FIG. 11B, the surface of thesingle crystal manufactured under atmospheric pressure condition was farflat, and although lines indicating the molecular steps having a stepdifference of 0.5 nm are observed, the number of lines is much smallerthan that observed in the single crystal sample manufactured underreduced pressure condition. That is, it demonstrates that the surface ofthe single crystal exhibiting large multiplication has a great number ofmolecular step structures, and the surface of the single crystalexhibiting almost no multiplication has a small number of molecular stepstructures.

From these observations, existence of a great number of molecular stepsin which molecular surface gets disconnected became clear as a higherdimensional structure which is larger than the molecular arrangement. Upto now, observation of such ultrafine structure in a vapor-depositedfilm was completely impossible, and has never succeeded. This is firstenabled by using a single crystal.

The mechanism of multiplication phenomenon in single crystal issubstantially equal to the multiplication behavior of the NTCDA thinfilm manufactured by vacuum vapor deposition, and as explained withreference to FIG. 1, it is based on the light-induced electron injectionmechanism.

FIG. 12 shows the structure of molecular level at the interface betweenthe NTCDA crystal and Au in actual scale. The line segment on the upperright represents 10 nm, and the arrangement of short line segmentsdepicted on the upper right portion of the crystal shows the arrangementof NTCDA molecules, and each line segment represents an NTCDA molecule.The ball on the right side (depicted by circle) represents an Aunanonarticle.

It has already been found from AFM observation that the Auvapor-deposited electrode is an assembly of ultrafine particles, andthere is a spatial gap between the Au spherical particle of about 20 nmin diameter and the surface of the NTCDA crystal. And the molecularsteps of about 0.5 nm form a number of dead-end roads. Strong electricfield concentrates at the interface when multiplication occurs, andholes need to counter the electric field for going out of the moleculardead-end road (arrow in the vicinity of the center of the drawing).Consequently, the holes become hard to escape to the Au electrode andaccumulate to cause multiplication.

There is a clear relationship between crystallinity and multiplicationbehavior of an organic vapor-deposited thin film. However explanationfor this experimental evidence had not been given. FIG. 13 showsdependences on applied voltage of multiplication factor exhibited by thecrystalline vapor-deposited thin film (Me-PTC) and the amorphousvapor-deposited thin film (PhEt-PTC). The crystalline thin film exhibitslarge multiplication, while the amorphous thin film exhibits almost nomultiplication. This relationship is established in all organic thinfilms examined by the inventors without exception. By way of themolecular dead-end model shown in FIG. 12, this fact can be reasonablyexplained. In other words, since there is no regular moleculararrangement and obviously no molecular step dead-end structure in theamorphous thin film, multiplication will not occur. To the contrary,since the crystalline vapor-deposited thin film is an assembly of finecrystals, there would be a larger number of molecular step dead-endstructures on the microcrystal surface than in the case of the singlecrystal, so that large multiplication will occur. From thisconsideration, it can be concluded that the molecular dead-end structureshown in FIG. 12 is essential for accumulation of holes at theorganic/metallic interface.

Although the structural trap model of FIG. 2 was a view perfectly drawnin the mind's eye, FIG. 12 is a view depicted based on the actual AFMobservation.

The essence of structural trap is:

(i) there is a gap at an metallic/organic interface; and

(ii) there is a dead-end road structure.

Both of these are essential for accumulation of holes at theorganic/metallic interface.

From this essence of structural trap, it is clear that the larger thenumber of molecular step structures which are to be molecular dead-endroads, the larger multiplication is exhibited. The above experimentalresults (FIG. 8 and FIG. 11) directly show this fact.

This result shows that it is possible to control multiplicationcharacteristics by controlling or even designing the molecular stepstructure of the organic/metallic interface.

Next, faster multiplication response which is a notable characteristicof the present invention shall be explained. As one example, FIGS. 15Aand 15B show a comparison of response characteristics at the sameelectric field intensity between the NTCDA single crystal and thevapor-deposited film. FIG. 15A shows the case where the single crystalwas used, and FIG. 15B shows the case where the vapor-deposited film wasused. In each graph, the vertical axis represents electric current (μA)and the horizontal axis represents time (sec.). It can be seen that theleading edge at the time of light-on is significantly sharper in thecase where the single crystal was used than the case where thevapor-deposited film was used. This shows that the single crystal makesthe response faster regardless of the associated disadvantage conditionthat the thickness of photoelectric current multiplier layer is muchlarger than that of the vapor-deposited film. This may result from thefact that the degree of carrier transfer of the single crystal is largerthan that of the vapor-deposited film.

As described above, use of an organic semiconductor single crystalenables flexible control of multiplication characteristics via controlof the density of molecular steps.

As methods of controlling the molecular step density, the followingmethods can be exemplified.

(1) Method of controlling the single crystal manufacturing condition asshown in the above embodiments.

(2) Method of vapor-depositing the same organic semiconductor on theorganic semiconductor single crystal while controlling the temperatureof substrate, to allow epitaxial growth. In this case, since the samemolecules are used, the problem of lattice mismatch will not occur andcontrol of structure is relatively easy. When this method is conductedusing a molecular beam epitaxial (MBE) apparatus, more accurate control,such as control of a monomolecular layer can be realized.

(3) Method of cutting at various angles using a microtome. Since thestep density differs depending on the crystal surface, it is possible tocreate a variety of exposed crystal surfaces so that the step densitycan be controlled by cutting at various angles using a microtome.

(4) Method of heat treating the single crystal. Also by heat treatmentof the single crystal, it is possible to control the step density.

(5) Method of directly processing the surface of the single crystal byusing AFM probe.

(6) Method of processing the surface of the single crystal by ionbombardment.

(7) Method of processing the single crystal by etching with an organicsolvent or vapor of organic solvent.

Next, an embodiment of the above mentioned method (2) shall beexplained.

On a substrate of NTCDA single crystal which was manufactured underatmospheric pressure condition and showed little multiplication in theabove embodiment, NTCDA was vapor-deposited at room temperature inthickness of 30 nm, to prepare a sample.

In FIG. 16, (c) is the dependence on applied voltage of multiplicationfactor exhibited by a device that is produced by using the samplewherein NTCDA is vapor-deposited on the surface of the NTCDA singlecrystal substrate in this manner. Also in FIG. 16, (b) is, likewiseshown as (b) in FIG. 8, dependence on applied voltage of multiplicationfactor exhibited by a device that is produced by using the NTCDA singlecrystal manufactured under atmospheric pressure condition. In comparingthe results of (b) and (c), the multiplication factor of (c) is clearlylarger than that of (b).

FIG. 17 shows an AFM image of the surface of the above sample which isobtained by vapor-depositing NTCDA on the substrate of NTCDA singlecrystal manufactured under atmospheric pressure condition at roomtemperature so as to have a thickness of 30 nm. Fine crystals of NTCDAgenerated by vapor-deposition can be observed, and more molecular stepsare possessed compared to the flat single crystal surface shown in FIG.11( b).

The molecular steps on the surface of the NTCDA single crystal containnot only straight lines but also curved portions as shown in FIG. 18A.FIG. 18A shows an AFM image of 200 nm×200 nm area on the surface. Bygradually enlarging such a curved portion, an image of molecular crystalshown in FIG. 18B appears. Those denoted by short parallel line segmentsin FIG. 18B are NTCDA molecular crystals, and have a size correspondingto that of the NTCDA molecule schematically shown in FIG. 18C.

FIG. 18B is an enlarged view of a part of FIG. 18A in the samedirection. However, the direction of arrangement of molecular steps inFIG. 18A and the direction of arrangement of NTCDA molecular crystals inFIG. 18B do not coincide. This is because there are many kinkingportions where the arrangement of NTCDA molecular crystals as shown inFIG. 18B bends, and hence when viewing a larger range as shown in FIG.18A, the arrangement directions of molecular steps are bent. Also thepossibility that such kinking portion plays an important role incapturing holes, namely, the possibility that holes are trapped as ifthey get tangled on the kinking portion can be considered. Ultimately,it is possible to control multiplication characteristics by controllingnot only steps but also such kinking structure.

The single crystal of the organic semiconductor used in the presentinvention may be generally applied to any organic semiconductors capableof manufacturing single crystals, as well as NTCDA exemplified in theembodiment.

Although only multiplication factor is shown as multiplicationcharacteristics in the embodiment, response speed of multiplication maybe controlled by controlling the molecular step structure at interface.

As described above, according to the present invention, since a singlecrystal of an organic pigment was used as a photoconductive organicsemiconductor in a photoelectric current multiplier which obtains alight irradiation-induced current or light-to-light transduced lightwith multiplied quantum yield by irradiating a photoelectric currentmultiplier layer with light while a voltage is applied to thephotoelectric current multiplier layer containing a photoconductiveorganic semiconductor, it is possible to achieve the effect of improvingthe response speed.

As described above, according to the present invention, sincephotoelectric current multiplying characteristics are controlled viacontrol of molecular step structure on the surface of the organicsemiconductor single crystal used in the photoelectric currentmultiplier, it is possible to realize a photoelectric current multiplierof molecular crystal having an unprecedented characteristics or desiredcharacteristics.

Industrial Applicability

The photoelectric current multiplier according to the present inventioncan be used as a light detection device, and is especially suited fordetecting weak light. The light-to-light transducer is suited for use inthe fields that require wavelength conversion of light. Therefore, thepresent invention finds its application in wide fields such as opticalsensors, optical computers, displays and the like.

1. A photoelectric current multiplier comprising electrodes for applying a voltage to a photoelectric current multiplier layer containing a photoconductive organic semiconductor, wherein the photoconductive organic semiconductor is formed of a single crystal of an organic pigment; the photoelectric current multiplier layer obtains a light irradiation-induced current with a quantum yield multiplied by 1-fold or more by light irradiation under the condition that a voltage is applied by the electrodes; the photoelectric current multiplier layer is controlled in multiplication characteristics via control of molecular step structure at a metallic/organic interface between the photoconductive organic semiconductor and at least one of the electrodes being in contact therewith; and the photoelectric current multiplier layer realizes faster response than a photoelectric current multiplier comprising a vapor-deposited film formed of the same photoconductive organic semiconductor material.
 2. The photoelectric current multiplier according to claim 1, wherein the organic pigment is one kind selected from the group consisting of phthalocyanine-based pigments, quinacridone-based pigments, perylene-based pigments, naphthalene derivatives, pentacene and its derivatives, porphyrin and its derivatives, merocyanine and its derivatives, and fullerene.
 3. The photoelectric current multiplier having fast response according to claim 2, wherein the organic pigment is a naphthalene tetracarboxylic anhydride belonging to a naphthalene derivative.
 4. The photoelectric current multiplier having fast response according to claim 1, wherein the electrodes for applying a voltage to the photoelectric current multiplier layer are directly formed on the photoelectric current multiplier layer rather than supported by a substrate.
 5. The photoelectric current multiplier according to claim 1, wherein the photoelectric current multiplier layer includes an organic electroluminescent layer laminated and integrated therewith, thereby light-to-light converted light is obtained from the organic electroluminescent layer by irradiating the photoelectric current multiplier layer with light. 